Operational transconductance amplifier and filter circuit

ABSTRACT

The operational transconductance amplifier comprises a MOS field-effective transistor that controls the mutual conductance. The central voltage measurement circuit and the voltage addition circuit shift a gate voltage of the MOS field-effective transistor by an amount equal to the deviation of a source voltage of the MOS field-effective transistor caused by an input offset voltage Voff.

FIELD OF THE INVENTION

[0001] The present invention relates to an operational transconductanceamplifier (“OTA”) which is an amplifier having a mutual conductancewhich can be controlled, and a filter circuit which uses the OTA. Moreparticularly, this invention relates to the OTA and the filter circuitformed of complementary metal-oxide semiconductor (“CMOS”) devices.

BACKGROUND OF THE INVENTION

[0002] The mutual conductance of the OTA depends on the input offsetvoltage. If a circuit is formed of a plurality of OTA's, therefore,dispersion of the circuit becomes great as a result of changes in mutualconductances of the OTS's. In order to prevent such dispersion, it isdesirable to adopt a configuration in which the mutual conductance ofeach OTA is least affected by the input offset voltage.

[0003] Conventionally, as an OTA formed of CMOS devices, an OTA having aconfiguration shown in FIG. 1 is known. As shown in FIG. 1, this OTAincludes three N-channel MOSFET's (“NMOS field-effective transistors”)11, 12 and 13, four current sources 14, 15, 16 and 17, two inputterminals 18 and 19, two output terminals 20 and 21, and a controlvoltage input terminal 22.

[0004] A first NMOS field-effective transistor 11 is connected at itsgate to a first input terminal 18, to which an input voltage Vin isapplied. The first NMOS field-effective transistor 11 is connected atits drain to a second output terminal 20, which outputs an outputcurrent IoutX. In addition, the first NMOS field-effective transistor 11is connected at its drain to a power source terminal as well via a firstcurrent source 14. The first NMOS field-effective transistor 11 isconnected at its source to a third NMOS field-effective transistor 13 atits source, and to ground via a third current source 16.

[0005] A second NMOS field-effective transistor 12 is connected at itsgate to a second input terminal 19, to which an input voltage VinX isapplied. The second NMOS field-effective transistor 12 is connected atits drain to a first output terminal 21, which outputs an output currentIout. In addition, the second NMOS field-effective transistor 12 isconnected at its drain to a power source terminal as well via a secondcurrent source 15. The second NMOS field-effective transistor 12 isconnected at its source to the third NMOS field-effective transistor 13at its drain, and to the ground via a fourth current source 17. Thethird NMOS field-effective transistor 13 is connected at its gate to acontrol voltage input terminal 22, to which a control voltage Vc isapplied from the outside.

[0006] In this conventional OTA, mutual conductance is controlled byadjusting the control voltage Vc and thereby changing the resistance ofthe third NMOS field-effective transistor 13. Mutual conductance Gm ofthe OTA can be represented by using a gate-source voltage Vgs and athreshold voltage Vth of the third NMOS field-effective transistor 13and a transconductance factor K as indicated by the following equation(1).

Gm=K(Vgs−Vth)  (1)

[0007] Typically, in the OTA, an input offset voltage of approximatelyseveral tens mV exists. Therefore, the voltage of the source of thethird NMOS field-effective transistor 13 rises by a voltagecorresponding to the input offset voltage. Denoting the input offsetvoltage by Voff, the equation (1) changes to the following equation (2).In other words, the conventional OTA has a problem that the mutualconductance is shifted from its preset value by −KVoff when the inputoffset voltage is Voff. $\begin{matrix}\begin{matrix}{{Gm} = {K\left( {{Vgs} - {Voff} - {Vth}} \right)}} \\{= {{K\left( {{Vgs} - {Vth}} \right)} - {Voff}}}\end{matrix} & (2)\end{matrix}$

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide an OTA inwhich the deviation of the mutual conductance caused by the input offsetvoltage is nearly zero, and provide a filter circuit that uses suchOTA's.

[0009] The operational transconductance amplifier according to oneaspect of the present invention comprises a MOS field-effectivetransistor having a gate, wherein a resistance of the MOSfield-effective transistor changes according to a voltage applied to thegate and transconductance is changed according to the changedresistance, a center voltage measurement circuit which measures a centervoltage of two input voltages and outputs one of a voltage and a currentbased on the measured center voltage, and a voltage addition circuitwhich supplies the gate of the MOS field-effective transistor with avoltage obtained by adding the voltage or current output from the centervoltage measurement circuit to a control voltage or a control current.

[0010] The operational transconductance amplifier according to anotheraspect of the present invention comprises a MOS field-effectivetransistor having a gate, wherein a resistance of the MOSfield-effective transistor changes according to a voltage applied to thegate and transconductance is changed according to the changedresistance, a first voltage addition circuit which outputs one of avoltage and a current obtained by adding a voltage or current based on afirst input voltage to a control voltage or a control current, a secondvoltage addition circuit which outputs one of a voltage and a currentobtained by adding a voltage or current based on a second input voltageto the control voltage or the control current, and a center voltagemeasurement circuit which measures a center voltage of the voltage orthe current output from the first voltage addition circuit and thevoltage or the current output from the second voltage addition circuit,and which supplies a voltage based on the measured center voltage to thegate of the MOS field-effective transistor.

[0011] The operational transconductance amplifier according to stillanother aspect of the present invention comprises a plurality of MOSfield-effective transistors each having a gate, wherein a resistance ofeach of the MOS field-effective transistors changes according to avoltage applied to the gate and transconductance is changed according tothe changed resistance, wherein the MOS field-effective transistorsbeing connected so that resistors having the same resistance will beconnected in series according to the gate voltage, and which output oneof a voltage and a current based on a center voltage of two inputvoltages, from a node of the resistors connected in series, and avoltage addition circuit which supplies the gates of the MOSfield-effective transistors with a voltage obtained by adding thevoltage or current output from the node of the MOS field-effectivetransistors to a control voltage or a control current.

[0012] The filter circuit according to still another aspect of thepresent invention comprises a plurality of operational transconductanceamplifiers according to the above-mentioned aspects.

[0013] Other objects and features of this invention will become apparentfrom the following description with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a circuit diagram which shows a configuration of aconventional OTA,

[0015]FIG. 2 is a principle diagram which shows a first configuration ofan OTA according to the present invention,

[0016]FIG. 3 is a principle diagram which shows a second configurationof an OTA according to the present invention,

[0017]FIG. 4 is a principle diagram which shows a third configuration ofan OTA according to the present invention,

[0018]FIG. 5 is a circuit diagram which shows a configuration of an OTAaccording to a first embodiment of the present invention,

[0019]FIG. 6 is a characteristic diagram which shows a verificationresult of the OTA shown in FIG. 5,

[0020]FIG. 7 is a circuit diagram which shows a configuration of abi-quad circuit used in the verification,

[0021]FIG. 8 is a table which shows a result of simulation conducted onthe bi-quad circuit of FIG. 7 formed by using the OTA shown in FIG. 5,

[0022]FIG. 9 is a table which shows a result of simulation conducted onthe bi-quad circuit of FIG. 7 formed by using a conventional OTA shownin FIG. 1,

[0023]FIG. 10 is a circuit diagram which shows a configuration of an OTAaccording to a second embodiment of the present invention,

[0024]FIG. 11 is a circuit diagram which shows another configuration ofan OTA according to the second embodiment of the present invention, and

[0025]FIG. 12 is a circuit diagram which shows a configuration of an OTAaccording to a third embodiment of the present invention.

DETAILED DESCRIPTIONS

[0026] According to the present invention, the object is achieved byshifting a gate voltage of a field-effective transistor, which is usedto control the mutual conductance, by a magnitude equivalent todeviation of a source voltage caused by an input offset voltage, therebypreventing a gate-source voltage from changing, and suppressing thedeviation of the mutual conductance to nearly zero. When the gatevoltage is shifted according to a change of the source voltage caused bythe input offset voltage, the change V1 of the gate voltage isrepresented by V1=Voff and the mutual conductance Gm is given by thefollowing equation (3). Therefore, it will be appreciated that themutual conductance is not affected by the input offset. $\begin{matrix}\begin{matrix}{{Gm} = {K\left( {{Vgs} - {Voff} - {Vth} + {V1}} \right)}} \\{= {K\left( {{Vgs} - {Vth}} \right)}}\end{matrix} & (3)\end{matrix}$

[0027] In order to shift the gate voltage by a magnitude equivalent tothe deviation of the source voltage as explained above, the followingconfiguration is employed. The same components as those of theconventional OTA shown in FIG. 1 are denoted by like characters, anddescription thereof will be omitted. FIG. 2 is a principle diagram whichshows a first configuration of an OTA according to the presentinvention. This OTA has a configuration such that a center voltagemeasurement circuit 3 and a voltage addition circuit 4 are added theconventional OTA. As already explained above, the conventional OTA hasthree NMOS field-effective transistors 11, 12 and 13, four currentsources 14, 15, 16 and 17, two input terminals 18 and 19, two outputterminals 20 and 21, and a control voltage input terminal 22. The centervoltage measurement circuit 3 outputs a center voltage of two voltagesrespectively applied to two input terminals 18 and 19. This centervoltage changes by the input offset voltage. In other words, if theinput voltage rises by the input offset voltage is Voff, then the outputvoltage of the center voltage measurement circuit 3 also rises by Voff.

[0028] The voltage addition circuit 4 has a source follower, whichincludes a voltage controlled current source 41, and a P-channelMOSFET's (“PMOS field-effective transistors”) 42, which receives theoutput of the center voltage measurement circuit 3 as its gate input. Anoutput of the voltage addition circuit 4 is supplied to a third NMOSfield-effective transistor 13 at its gate as a control voltage Vc. Thevoltage addition circuit 4 is formed of the source follower. If theoutput voltage of the center voltage measurement circuit 3, i.e., theinput of the source follower rises by Voff, therefore, the controlvoltage Vc output from the voltage addition circuit 4 also rises byVoff. In other words, in case of the third NMOS field-effectivetransistor 13, even if the source voltage deviates by Voff as a resultof the input offset, the gate voltage also deviates by Voff, andconsequently the gate-source voltage does not change. Therefore, themutual conductance does not change. The control voltage Vc is controlledby a current source control voltage Vic, which is input from the outsideto a current source control voltage input terminal 23 in order tocontrol the voltage controlled current source 41.

[0029]FIG. 3 is a principle diagram which shows a second configurationof an OTA according to the present invention. This OTA is obtained byadding a first voltage addition circuit (voltage addition circuit 1) 5,a second voltage addition circuit (voltage addition circuit 2) 6, and acenter voltage measurement circuit 7 into the conventional OTA. Asalready explained above, the conventional OTA has three NMOSfield-effective transistors 11, 12 and 13, four current sources 14, 15,16 and 17, two input terminals 18 and 19, two output terminals 20 and21, and a control voltage input terminal 22.

[0030] The first voltage addition circuit 5 has a source follower, whichincludes a voltage controlled current source 51, and a PMOSfield-effective transistor 52, which receives an input of the firstinput terminal 18 as its gate input. The second voltage addition circuit6 has a source follower, which includes a voltage controlled currentsource 61, and a PMOS field-effective transistor 62, which receives aninput of the second input terminal 19 as its gate input. The centervoltage measurement circuit 7 supplies a center voltage of two voltagesoutput from the first and second voltage addition circuits 5 and 6 to athird NMOS field-effective transistor 13 at its gate as a controlvoltage Vc.

[0031] Each of the two voltage addition circuits 5 and 6 is formed ofthe source follower. If the input voltage rises by the offset voltageVoff, therefore, the output voltage of the two voltage addition circuits5 and 6 also rises by Voff. Therefore, the control voltage Vc outputfrom the center voltage measurement circuit 7 also rises by Voff. Inother words, as regards the third NMOS field-effective transistor 13,even if the source voltage deviates by Voff as a result of the inputoffset, the gate voltage also deviates by Voff, and consequently thegate-source voltage does not change. Therefore, Gm does not change. Thecontrol voltage Vc is controlled by a current source control voltageVic, which is input from the outside to a current source control voltageinput terminal 23 in order to control the voltage controlled currentsources 51 and 61.

[0032]FIG. 4 is a principle diagram which shows a third configuration ofan OTA according to the present invention. This OTA is obtained byreplacing the third NMOS field-effective transistor 13, of the OTA shownin FIG. 2, which receives the control voltage Vc as the gate input, withtwo NMOS field-effective transistors 81 and 82 having the samecharacteristic and providing the NMOS field-effective transistors 81 and82 with the mutual conductance control function using the controlvoltage Vc and the function of the center voltage measurement circuit.In other words, resistance of each of the NMOS field-effectivetransistors 81 and 82 depends upon the control voltage Vc input to itsgate, and the mutual conductance changes according to the change of theresistance.

[0033] If the input voltage rises by the input offset voltage Voff, avoltage at a node N1 between the NMOS field-effective transistors 81 and82 rises by Voff in keeping therewith. The voltage at the node N1becomes an input of a source follower of a voltage addition circuit 4.If the voltage at the node N1 rises by voltage Voff, an output of thesource follower, i.e., the control voltage Vc also rises by Voff inkeeping therewith. In other words, in each of the NMOS field-effectivetransistors 81 and 82 used instead of the third NMOS field-effectivetransistor 13, both its source voltage and gate voltage deviate by Voffand consequently the gate-source voltage does not change. Therefore, Gmdoes not change.

[0034] Embodiments of the present invention will be described in detailbelow while referring to the accompanying drawings.

[0035]FIG. 5 is a circuit diagram which shows a configuration of an OTAaccording to a first embodiment of the present invention. The firstembodiment is an embodiment of the OTA of the first configuration shownin FIG. 2. In FIG. 5, therefore, the same components as those in FIG. 2are denoted by like characters with those of FIG. 2.

[0036] In the first embodiment, the third NMOS field-effectivetransistor 13 is connected at its source and drain between sources ofthe first NMOS field-effective transistor 11 and the second NMOSfield-effective transistor 12 as shown in FIG. 5. The third NMOSfield-effective transistor 13 receives a control voltage Vc as its gateinput. The resistance of the third NMOS field-effective transistor 13changes according to the magnitude of the control voltage Vc. Thus, themutual conductance can be controlled by changing the resistance of thethird NMOS field-effective transistor 13. The first NMOS field-effectivetransistor 11 receives the input voltage Vin as its gate input, andoutputs the output current IoutX as its drain current. The second NMOSfield-effective transistor 12 receives the input voltage VinX as itsgate input, and outputs the output current Iout as its drain current.

[0037] The center voltage measurement circuit 3 includes two resistors31 and 32 having the same resistance, two NMOS field-effectivetransistors 33 and 34, and four current sources 35, 36, 37 and 38. Theresistors 31 and 32 are connected in series between sources of thefourth NMOS field-effective transistor 33 and the fifth NMOSfield-effective transistor 34. Voltage at a node of the two resistors 31and 32 is supplied to a voltage addition circuit 4 as an output of thecenter voltage measurement circuit 3.

[0038] The fourth NMOS field-effective transistor 33 is connected at itsgate to a first input terminal 18, to which an input voltage Vin isapplied. The fourth NMOS field-effective transistor 33 is connected atits drain to a power source terminal via a fifth current source 35. Thefourth NMOS field-effective transistor 33 is connected at its source toground via a seventh current source 37. The fifth NMOS field-effectivetransistor 34 is connected at its gate to a second input terminal 19, towhich the input voltage VinX is applied. The fifth NMOS field-effectivetransistor 34 is connected at its drain to a power source terminal viaan eighth current source 38. The fifth NMOS field-effective transistor34 is connected at its source to ground via the eighth current source38.

[0039] The voltage addition circuit 4 has a source follower including aPMOS field-effective transistor 43, which forms a voltage controlledcurrent source 41, and a PMOS field-effective transistor 42, whichreceives the output of the center voltage measurement circuit 3 as itsgate input. An output of the source follower is supplied to the gate ofthe third NMOS field-effective transistor 13 as the control voltage Vc.The PMOS field-effective transistor 43 forming the voltage controlledcurrent source 41 receives a current source control voltage Vic suppliedfrom the outside as its gate input, and controls the magnitude of thecontrol voltage Vc according to the current source control voltage Vic.

[0040] If the input voltage changes by the input offset voltage Voff inthe OTA having the configuration shown in FIG. 5, then the sourcevoltage of the third NMOS field-effective transistor 13 changes by Voff.Further, if the input voltage changes by the input offset voltage Voff,then the output voltage of the center voltage measurement circuit 3changes by Voff. The voltage addition circuit 4 is formed of the sourcefollower as explained above. If the input voltage of the source followersupplied from the center voltage measurement circuit 3 changes by Voff,therefore, the control voltage Vc output from the voltage additioncircuit 4 changes by Voff. Since the control voltage Vc is the gatevoltage of the third NMOS field-effective transistor 13, both the gatevoltage and the source voltage of the third NMOS field-effectivetransistor 13 change by Voff. Accordingly, the gate-source voltage ofthe third NMOS field-effective transistor 13 does not change whetherVoff is present or not and whether Voff is large or small. Therefore, Gmof the OTA does not change.

[0041] Two verifications were made by the present inventors in order toascertain the effectiveness of the OTA having the configuration shown inFIG. 5. The OTA shown in FIG. 5 and the conventional OTA shown in FIG. 1were compared. In a first verification, the input voltages Vin and VinXof each OTA is swept in the same direction in the range of −0.08 V to0.08 V assuming that offset voltages are applied thereto, and a changeof the mutual conductance Gm is checked.

[0042] The technology is 0.35 μm, and the power source voltage is 2.8 V.A result of the first verification is shown in FIG. 6. As apparent fromFIG. 6, the change of the mutual conductance Gm in the sweep range isnearly zero in the OTA of the first embodiment, whereas the mutualconductance changes greatly in the OTA of the conventional art. As aresult, it can be ascertained that the deviation of the mutualconductance can be suppressed to nearly zero in the first embodimenteven if there is an input offset voltage.

[0043] As a second verification, band pass filters formed ofsecond-order bi-quad circuits having a configuration shown in FIG. 7 arefabricated by using the OTA shown in FIG. 5 and the conventional OTAshown in FIG. 1 serving as the subject of comparison, and the MonteCarlo simulation of SPICE is conducted to check dispersion. A centerfrequency of the band pass filter is 450 kHz, and an attenuation at450±50 kHz is 10 dB. In FIG. 7, OTA2, OTA3, OTA4 and OTA5 are OTA'S, andCa and Cb are capacitors.

[0044] Simulation results of the band pass filter using the OTA shown inFIG. 5 and the band pass filter using the OTA shown in FIG. 1 are shownin FIGS. 8 and 9, respectively. In the band pass filter using the OTA ofthe present first embodiment, 3δ of an attenuation at a space of −50 kHzis 0.26 and 3δ of an attenuation at a space of +50 kHz distance is 0.23(see FIG. 8). On the other hand, in the band pass filter using the OTAof the conventional art, 3δ of an attenuation at −50 kHz distance is0.37 and 3δ of an attenuation at +50 kHz distance is 0.32. Both of themare greater as compared with the band pass filter using the OTA of thepresent first embodiment. In other words, it has been confirmed that thecircuit using the OTA of the present first embodiment is smaller indispersion than the circuit using the OTA of the conventional art.

[0045] In the first embodiment, the gate voltage of the third NMOSfield-effective transistor 13 used to control the mutual conductance Gmis shifted by a magnitude equivalent to the deviation of the sourcevoltage caused by the input offset voltage. Accordingly, the gate-sourcevoltage of the third NMOS field-effective transistor 13 is preventedfrom changing. The deviation of the mutual conductance Gm is thussuppressed to nearly zero. As a result, an OTA in which the deviation ofGm caused by the input offset voltage is nearly zero and a filtercircuit using such OTA's are obtained.

[0046]FIG. 10 is a circuit diagram which shows a configuration of an OTAaccording to a second embodiment of the present invention. The secondembodiment is an embodiment of the OTA of the second configuration shownin FIG. 3. In FIG. 10, therefore, the same components as those in FIG. 3are denoted by like characters with those of FIG. 3. In the secondembodiment, the basic configuration of the OTA including the first NMOSfield-effective transistor 11, the second NMOS field-effectivetransistor 12 and the third NMOS field-effective transistor 13 is thesame as that of the first embodiment. Moreover, the mutual conductanceis controlled by changing the resistance of the third NMOSfield-effective transistor 13 in the same as the first embodiment.

[0047] The first voltage addition circuit 5 has a source followerincluding a PMOS field-effective transistor 53, which forms a voltagecontrolled current source 51, and the PMOS field-effective transistor52, which receives an input voltage Vin as its gate input. The PMOSfield-effective transistor 53 forming the voltage controlled currentsource 51 receives a current source control voltage Vic supplied fromthe outside as its gate input, and supplies a voltage according to thecurrent source control voltage Vic to the center voltage measurementcircuit 7. The second voltage addition circuit 6 has a source followerincluding a PMOS field-effective transistor 63, which forms a voltagecontrolled current source 61, and the PMOS field-effective transistor62, which receives an input voltage VinX as its gate input. The PMOSfield-effective transistor 63 forming the voltage controlled currentsource 61 receives a current source control voltage Vic supplied fromthe outside as its gate input, and supplies a voltage according to thecurrent source control voltage Vic to the center voltage measurementcircuit 7.

[0048] The center voltage measurement circuit 7 includes two resistors71 and 72 having the same resistance. These two resistors 71 and 72 areconnected in series between an output terminal of the source follower ofthe first voltage addition circuit 5 and an output terminal of thesource follower of the second voltage addition circuit 6. Voltage at anode of these two resistors 71 and 72 is supplied to the gate of thethird NMOS field-effective transistor 13 as the control voltage Vc.

[0049] If the input voltage changes by the input offset voltage Voff inthe OTA having the configuration shown in FIG. 10, then the sourcevoltage of the third NMOS field-effective transistor 13 changes by Voff.Further, if the input voltage changes by the input offset voltage Voff,then each of the output voltage of the first voltage addition circuit 5and the output voltage of the second voltage addition circuit 6 alsochanges by Voff, because each of the output voltage of the first voltageaddition circuit 5 and the output voltage of the second voltage additioncircuit 6 is formed of a source follower as explained above.Accordingly, the control voltage Vc output from the center voltagemeasurement circuit 7 also changes by Voff. Therefore, both the gatevoltage and the source voltage of the third NMOS field-effectivetransistor 13 change by Voff. Accordingly, the gate-source voltage ofthe third NMOS field-effective transistor 13 does not change whetherVoff is present or not and whether Voff is large or small. Therefore, Gmof the OTA does not change.

[0050] In the second embodiment, the gate voltage of the third NMOSfield-effective transistor 13 used to control the mutual conductance Gmis shifted by a value equal to the deviation of the source voltagecaused by the input offset voltage. Accordingly, the gate-source voltageof the third NMOS field-effective transistor 13 is prevented fromchanging. The deviation of the mutual conductance Gm is thus suppressedto nearly zero. As a result, an OTA in which the deviation of Gm causedby the input offset voltage is nearly zero and a filter circuit usingsuch OTAs are obtained.

[0051] A similar effect is also obtained in a configuration shown inFIG. 11. In the configuration shown in FIG. 11, the source voltage ofthe first NMOS field-effective transistor 11, which receives the inputvoltage Vin as the gate input, is used instead of the input voltage Vinas the gate input of the PMOS field-effective transistor 52 forming thesource follower of the first voltage addition circuit 5, and the sourcevoltage of the second NMOS field-effective transistor 12, which receivesthe input voltage VinX as the gate input, is used as the gate input ofthe PMOS field-effective transistor 62 forming the source follower ofthe second voltage addition circuit 6.

[0052]FIG. 12 is a circuit diagram which shows a configuration of an OTAaccording to a third embodiment of the present invention. The thirdembodiment is an embodiment of the OTA of the third configuration shownin FIG. 4. In FIG. 12, therefore, the same components as those in FIG. 4are denoted by like characters with those of FIG. 4. In the thirdembodiment, the third NMOS field-effective transistor 13 in the firstembodiment shown in FIG. 5 is replaced with two NMOS field-effectivetransistor, i.e., an NMOS field-effective transistor 81 and an NMOSfield-effective transistor 82 having the same characteristic, and thecontrol voltage Vc is supplied to gates of the NMOS field-effectivetransistors 81 and 82.

[0053] The NMOS field-effective transistor 81 is connected at its sourceto the source of the first NMOS field-effective transistor 11, whichreceives the input voltage Vin as the gate input. The NMOSfield-effective transistor 81 is connected at its drain to the NMOSfield-effective transistor 82 at its source. The NMOS field-effectivetransistor 82 is connected at its drain to the source of the second NMOSfield-effective transistor 12, which receives the input voltage VinX asthe gate input. In the third embodiment, the basic configuration of theOTA including the two NMOS field-effective transistors 81 and 82 insteadof the first NMOS field-effective transistor 11, the second NMOSfield-effective transistor 12 and the third NMOS field-effectivetransistor 13, and control the mutual conductance Gm is conducted bychanging the resistance of the NMOS field-effective transistors 81 and82 are the same as those of the first embodiment.

[0054] The NMOS field-effective transistors 81 and 82 have a function ofthe center voltage measurement circuit. Voltage at the node N1 of theNMOS field-effective transistors 81 and 82 is supplied to the gate ofthe PMOS field-effective transistor 42, which forms the source followerof the voltage addition circuit 4. The output of the source follower issupplied to gates of the NMOS field-effective transistors 81 and 82instead of the third NMOS field-effective transistor 13 as the controlvoltage Vc.

[0055] If the input voltage in the OTA having the configuration shown inFIG. 12 changes by the input offset voltage Voff, then a voltage of thesource of the NMOS field-effective transistor 81 connected to the sourceof the first NMOS field-effective transistor 11 changes by Voff, and avoltage of the drain of the NMOS field-effective transistor 82 connectedto the source of the second NMOS field-effective transistor 12 changesby Voff. Therefore, the voltage at the node N1 of the NMOSfield-effective transistors 81 and 82 also changes by Voff. This isequivalent to a Voff change of the source voltage of the NMOSfield-effective transistor 82 connected to the source of the second NMOSfield-effective transistor 12.

[0056] The voltage addition circuit 4 is formed of the source followeras explained above. If the voltage at the node N1, i.e., the inputvoltage of the source follower changes by Voff, therefore, the controlvoltage Vc output from the voltage addition circuit 4 changes by Voff.Since the control voltage Vc is the voltage applied to gates of the NMOSfield-effective transistors 81 and 82 provided instead of the third NMOSfield-effective transistor 13, both the gate voltage and the sourcevoltage of the NMOS field-effective transistors 81 and 82 change byVoff. Accordingly, the gate-source voltage of each of the NMOSfield-effective transistors 81 and 82 does not change whether Voff ispresent or not and whether Voff is large or small. Therefore, Gm of theOTA does not change.

[0057] In the third embodiment, the gate voltage of each of the NMOSfield-effective transistors 81 and 82 used to control the mutualconductance Gm is shifted by a value equal to the deviation of thesource voltage caused by the input offset voltage. Accordingly, thegate-source voltage of each of the NMOS field-effective transistors 81and 82 is prevented from changing. The deviation of the mutualconductance Gm is thus suppressed to nearly zero. As a result, an OTA inwhich the deviation of Gm caused by the input offset voltage is nearlyzero and a filter circuit using such OTAs are obtained.

[0058] In the third embodiment, the center voltage needs only be outputfrom the node N1. Therefore, the number of transistors that replace theNMOS field-effective transistor 13 need not be two (i.e., the NMOSfield-effective transistor 81 and 82). The number of transistors thatreplace the NMOS field-effective transistor 13 may be more than two.Moreover, all the transistors that replace the NMOS field-effectivetransistor 13 need not have the same resistance. Since the centervoltage needs only be output from the node N1, it is only necessary thatbilateral symmetry is obtained about N1 and the sum total of resistanceson the left hand of N1 is equal to that on the right hand.

[0059] To be more concrete, the mirror type is required in order toobtain correlation. To implement the mirror type, all of the followingconditions must be satisfied, the number of transistors located on theleft hand should be equal to the number of transistors located on theright hand; the sum total of resistance values of transistors located onthe left hand should be equal to the sum total of resistance values oftransistors located on the right hand; and if transistors located oneach of the left and right hands are formed of a plurality oftransistors having different resistance values, resistances ofcorresponding transistors located on the left hand and the right handshould be the same.

[0060] The configuration of the present invention can be modified invarious ways. The voltage addition circuit and the center voltagemeasurement circuit are not limited to the above-explainedconfiguration. Further, the OTA according to the present invention isnot limited to a filter circuit, but can be applied to various circuitsusing OTAs.

[0061] According to the present invention, the gate voltage of thetransistor used to control the mutual conductance Gm is shifted by amagnitude equivalent to the deviation of the source voltage caused bythe input offset voltage. Accordingly, the gate-source voltage isprevented from changing. The deviation of the mutual conductance Gm isthus suppressed to nearly zero. As a result, an OTA in which thedeviation of Gm caused by the input offset voltage is nearly zero and afilter circuit using such OTAs are obtained.

[0062] Although the invention has been described with respect to aspecific embodiment for a complete and clear disclosure, the appendedclaims are not to be thus limited but are to be construed as embodyingall modifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. An operational transconductance amplifier comprising: a MOS field-effective transistor having a gate, wherein a resistance of said MOS field-effective transistor changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance; a center voltage measurement circuit which measures a center voltage of two input voltages and outputs one of a voltage and a current based on the measured center voltage; and a voltage addition circuit which supplies said gate of said MOS field-effective transistor with a voltage obtained by adding the voltage or current output from said center voltage measurement circuit to a control voltage or a control current.
 2. The operational transconductance amplifier according to claim 1, wherein said voltage addition circuit comprises a source follower which in turn comprises at least one MOS field-effective transistor.
 3. The operational transconductance amplifier according to claim 1, wherein said voltage addition circuit comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside.
 4. An operational transconductance amplifier comprising: a MOS field-effective transistor having a gate, wherein a resistance of said MOS field-effective transistor changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance; a first voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a first input voltage to a control voltage or a control current; a second voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a second input voltage to the control voltage or the control current; and a center voltage measurement circuit which measures a center voltage of the voltage or the current output from said first voltage addition circuit and the voltage or the current output from said second voltage addition circuit, and which supplies a voltage based on the measured center voltage to said gate of said MOS field-effective transistor.
 5. The operational transconductance amplifier according to claim 4, wherein said first voltage addition circuit and said second voltage addition circuit each comprises a source follower which in turn comprises at least one MOS field-effective transistor.
 6. The operational transconductance amplifier according to claim 4, wherein said first voltage addition circuit and said second voltage addition circuit each comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside.
 7. An operational transconductance amplifier comprising: a plurality of MOS field-effective transistors each having a gate, wherein a resistance of each of said MOS field-effective transistors changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance, wherein said MOS field-effective transistors being connected so that resistors having the same resistance will be connected in series according to the gate voltage, and which output one of a voltage and a current based on a center voltage of two input voltages, from a node of said resistors connected in series; and a voltage addition circuit which supplies said gates of said MOS field-effective transistors with a voltage obtained by adding the voltage or current output from said node of said MOS field-effective transistors to a control voltage or a control current.
 8. The operational transconductance amplifier according to claim 7, wherein said voltage addition circuit comprises a source follower which in turn comprises at least one MOS field-effective transistor.
 9. The operational transconductance amplifier according to claim 7, wherein said voltage addition circuit comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside.
 10. A filter circuit comprising a plurality of operational transconductance amplifiers, each said operational transconductance amplifier having: a MOS field-effective transistor having a gate, wherein a resistance of said MOS field-effective transistor changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance; a center voltage measurement circuit which measures a center voltage of two input voltages and outputs one of a voltage and a current based on the measured center voltage; and a voltage addition circuit which supplies said gate of said MOS field-effective transistor with a voltage obtained by adding the voltage or current output from said center voltage measurement circuit to a control voltage or a control current, wherein said voltage addition circuit comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside.
 11. A filter circuit comprising a plurality of operational transconductance amplifiers, each said operational transconductance amplifier having: a MOS field-effective transistor having a gate, wherein a resistance of said MOS field-effective transistor changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance; a first voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a first input voltage to a control voltage or a control current; a second voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a second input voltage to the control voltage or the control current; and a center voltage measurement circuit which measures a center voltage of the voltage or the current output from said first voltage addition circuit and the voltage or the current output from said second voltage addition circuit, and which supplies a voltage based on the measured center voltage to said gate of said MOS field-effective transistor, wherein said first voltage addition circuit and said second voltage addition circuit each comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside.
 12. A filter circuit comprising a plurality of operational transconductance amplifiers, each said operational transconductance amplifier having: a plurality of MOS field-effective transistors each having a gate, wherein a resistance of each of said MOS field-effective transistors changes according to a voltage applied to said gate and transconductance is changed according to the changed resistance, wherein said MOS field-effective transistors being connected so that resistors having the same resistance will be connected in series according to the gate voltage, and which output one of a voltage and a current based on a center voltage of two input voltages, from a node of said resistors connected in series; and a voltage addition circuit which supplies said gates of said MOS field-effective transistors with a voltage obtained by adding the voltage or current output from said node of said MOS field-effective transistors to a control voltage or a control current, wherein said voltage addition circuit comprises at least one MOS field-effective transistor which serves as a voltage controlled current source which controls the control voltage or the control current based on a current source control voltage input from outside. 